Black Nitrate or Stainless Steel or other ? 2

[jdkuhndog]

I have an application where a wire rope drum will be exposed to wash downs of 10% molarity nitric acid on an intermittent basis. It will also be exposed to high level radiation.

My main concern is the corrosion resistance of my drum. We are trying to steer away from stainless due to price - I have heard a little bit (very little bit) about a process which adds some corrosion resistance to steel called Black Nitrating. Is this cost effective ?

I will have 10 of these wire rope drums. They will be approximately 40" diameter by 60" long. Most likely made out of 2" thick steel plate.

Anyhow, I was hoping to find out what exactly the black nitriding will do for me compared to stainless steel as far as corrosion is concerned and then to weigh that against the approximate costs for each.

I also have a question regarding which Grade of stainless steel would offer adequate corrosion resistance but also have considerable ductility and crushing strength to resist the stress induced by the lifted load via the wire rope.

Which brings up another issue...would there be any deleterious effects caused by carbon steel wire rope being in constant contact with stainless steel?

Thanks in advance for any responses to one or all of my questions !!

 

[hacksaw]

nitriding is the correct term. it is accomplished by immersion in a special salt baths. the orignal process has been replaced by newer methods.

That said I don't think that they provide resistance to nitric acid pickling. The pickling process usually imparts a degree of corrosion resistance on its own because is forms a thin oxide coating on the steel.

the greatest concern is for the other materials used on the drum i.e. the drum brake, these can be bronze.

 

[Metalguy]

Jdkuhndog,

Your application seems to be a good candidate for cladding carbon steel with stainless. It can be strip-clad using ~2" wide strip SS. Make sure they put 2 layers on, with the top layer overlapping the edges of the first layer.

Much cheaper than all SS construction. Use 309L for the 1st layer, and 309L or 308L for the 2nd-I'd want to stay with 309L for both (higher Cr).

If the wire rope is wet most of the time, it could corrode faster if its on a SS drum, because it will be the anode.

But *if* the conductivity of the water is very low (demin. water), it may not be a problem.

Hacksaw,

Pickling (I think you might mean passivating with nitric acid) helps SS, but not CS.

 

[jdkuhndog]

Thanks for the responses...

Not real familiar with cladding - this drum will be machined with grooves in it for the wire rope and machined on the ends for a barrel coupling - is it possible to galvanize or electroplate after machining is complete?

 

[Metalguy]

Yes, but the acid will destroy the zinc pretty fast, I think. Electroless nickel might work-where is the EN expert Kenvlach these days?

 

[hacksaw]

not so much passivation, pickling (cleaning)

 

[kenvlach]

“Black nitrating” – could be either nitriding (for wear resistance, as suggested by hacksaw) or a black oxide process, but neither adds much corrosion resistance.

“would there be any deleterious effects caused by carbon steel wire rope being in constant contact with stainless steel?” – Yes, so it is probably essential to have a hard material to avoid erosion-corrosion.

Cladding with a proper SS would be OK, but to protect all exposed surfaces and be flaw-free would probably be more expensive than solid SS. It could be thermally sprayed on to cope with the grooves.

Zn (galvanizing) or Ni plating would be useless, the nitric acid would ‘eat’ whatever wasn’t worn away by the wire rope.
Note to Metalguy: we use nitric acid (usually 50-100%) to strip EN.

I don’t believe any austenitic or ferritic SS would stand up to the wear, especially not an L grade (low carbon). Possible materials:
Martensitic SS
Precipitation Hardening SS (e.g., 17-4PH)
An alloyed, corrosion resistant cast iron.
a) High Si cast iron (~14.5% Si, ASTM A518, e.g, Duriron, Durichlor51, Superchlor)
b) High Cr cast iron (up to 25% Cr, ASTM A532) probably the best wear resistance, but more costly.
There are also Ni cast irons, but the Si & Cr types are more applicable and less expensive. A high Si cast iron is the most cost-effective for corrosion and wear resistance, and can handle up to 30% nitric acid. If chloride is present, use a high Si cast iron grade containing 4.5% Cr. This can resist the additional corrosive effects of the ferric chloride occurring due to the wire rope.
However, the high Si cast irons are brittle, and I’m not sure whether 4.5% Cr would give enough improvement, so maybe a high Cr cast iron is the best choice.

 

[jdkuhndog]

Thanks again for the responses...

The Customer's spec disallows any cast iron - I may be able to get an exception I doubt it though) if spheroidal (ductile) cast iron was specified on our end.

The 17-4PH is the SS we planned to utilize if we can not get away with anything cheaper.

Just want to make sure all understand that the nitric acid is 0.1M and is sprayed on less than once a week and then washed off with high pressure water. Is this still enough to corrode a carbon steel drum pretty quickly ? I think if we can show the drum to last 15 years we will be OK.

 

[Metalguy]

*If* you're sure you can rinse off all the acid (and quickly), go for the carbon steel. Time is the key-keep the very high corr. rate time short enough and the actual metal loss will be low.

But if someone forgets----!